1. Field of the Invention
This invention relates to a beam current limiting circuit for use in a CRT-type video projector.
2. Description of the Related Art
FIG. 11 shows a CRT beam current limiting circuit in a related art. In FIG. 11, SIG1 denotes a television signal, numeral 1 denotes a signal processing circuit, numeral 2 denotes an amplification circuit, 3R, 3G, and 3B denote CRTs for producing red display, green display, and blue display respectively, numeral 4 denotes a flyback transformer, numeral 5 denotes a current detection circuit, and numeral 6 denotes a comparison circuit for comparing a detected current with a reference value. The signal processing circuit 1 and the comparison circuit 6 are may be integrated as an integrated circuit IC1, but the operation is the same regardless of whether they are integrated or separate.
The operation is as follows: The television signal SIG1 is input to the signal processing circuit 1, which then makes image quality corrections to color, contrast, brightness, etc., and outputs a red signal R, a green signal G, and a blue signal B to the amplification circuit 2. The amplification circuit 2 amplifies the signal amplitude until each input signal becomes a voltage sufficient for driving the cathode of the corresponding CRT. For example, the amplified red signal is input to the cathode of the red CRT 3R. Consequently, a current IKR flows into the cathode. Likewise, a current IKG flows into the cathode of the green CRT 3G and a current IKB flows into the cathode of the blue CRT 3B.
The anode current of each CRT is introduced into the flyback transformer 4 and total anode current IA of the three CRTs flows into the primary terminal of the flyback transformer 4. The anode current IA is introduced into the current detection circuit 5 and is sent through a resistor R1 to a power supply VCC1. A beam current is detected as a potential drop across terminals of the detection resistor R1 based on the current IA and the current detection circuit 5 outputs a detected voltage VDET1 to the voltage comparison circuit 6, which then compares the VDET1 with a predetermined value. If the VDET1 is lower than the predetermined value, the voltage comparison circuit 6 sends an excessive current detection signal CONT6 to the signal processing circuit 1, which then changes the contrast and (or) brightness of the television signal, thereby lessening the average level of output R, G, and B signals. Consequently, the average level of the cathode currents IKR, IKG, and IKB is lessened and the anode current IA is limited.
If a CRT used with a video projector has a screen width across corner of 7 inches, a cathode current can be allowed to flow with up to an average value of about 600 μA or an instantaneous value of about 6000 μAp-p for use as the part alone. When the average value of the beam current for driving a CRT becomes excessive, the phosphor temperature rises, the light emission intensity lowers, and a CRT face plate is placed at a high temperature and is broken; this is a problem. For a power supply circuit for supplying a CRT anode current, if the current becomes excessive, each part circuit voltage fluctuates, distortion occurs on a television display screen, and the parts of the power supply circuit are broken due to overload; this is a problem. To prevent the problems from occurring, the CRT anode current is detected and is limited so that the current value becomes a predetermined value or less.
FIG. 12 shows the relationship between image signal input level and CRT anode current, and change in the input signal level with time. In the figure, a curve I1 is a line indicating the relationship between input signal level SPEAK and average anode current IA, a curve V1 is a line indicating a change in the input signal level with time, T1 denotes a horizontal blank period, T2 denotes a video period, S denotes the average level of video signal, and SPEAK denotes the maximum level of video. Even if SPEAK is constant, the average signal level fluctuates and thus S also fluctuates. Therefore, I1 is shown for a normal level image.
A point I1A on the curve I1 is a point indicating a current corresponding to a detected current level when current in a black image (cutoff current) is automatically adjusted in a configuration having an AKB (automatic kinescope bias) circuit not shown in the example, a point I1B is a point indicating a current when the maximum level of a standard signal is input, a point I1C is a point indicating a current level of the result of executing current limiting when a signal at a level larger than the standard level is input, and a point I1D is a point indicating a flowing current level if current limiting is not executed.
When a normal image signal is input, V1 is a waveform having asperities finely fluctuating in response to the image as shown in FIG. 12 and even if SPEAK is large, the average level S is smaller than it. Anode current has a characteristic of nonlinearly increasing in response to the signal level as shown by the curve I1; particularly if the input level is large, the anode current change ratio is large.
Current limiting is executed, whereby the current when a signal at a level larger than the standard signal is input is limited from the level of I1D to the level of I1C. As a result, as described above, the anode current as the total of three CRTs is limited to I1LIMIT.
FIG. 13 shows the relationship between signal level and anode current when a uniform plane image signal is input, and change in the input signal level with time. In the figure, a curve I2 is a line indicating the relationship between input signal level SPEAK and average anode current IA, a curve V2 is a line indicating a change in the input signal level with time. The average signal level of the plane image is high as indicated by V2 and thus the level of S increases near to SPEAK. Therefore, as compared with I1 in FIG. 12 when the normal average level image is input, the value of I2 is large even at the same input signal peak level, and thus reaches a current limit value I2LIMIT even if the input level is less than the standard signal level, as indicated by the point I2B in FIG. 13. I2LIMIT and the current level indicated by the point I2A in FIG. 13 correspond to and are the same values as I1LIMIT and the current level indicated by the point I1A in FIG. 12.
FIG. 14 is a three-axis graph to show the range in which the cathode currents of the three CRTs are limited. The axes represent the cathode currents IKR, IKG, and IKB. The plane having a triangle TRI1 touching the three axes as an outer periphery indicates the range in which the total of IKR, IKG, and IKB limited as the result of limiting the anode currents to I1LIMIT becomes the constant value I1LIMIT, and the triangular pyramid surrounded by the triangle and an origin O represents the range in which the three cathode currents can be changed.
A point P1 in the vicinity of the center of the triangle TRI1 is a point representing a current after current limiting circuit operates when a white plane signal or a normal image is input at a large level. IKR, IKG, and IKB corresponding to the point P1 take each roughly the value of a third of I1LIMIT. I1LIMIT is set to a value a little smaller than three times the rated average current value of a sole CRT beam. Consequently, the normal television signal or white plane signal is limited so that the beam current does not exceed the rated average current of each CRT, and an excessive load exceeding I1LIMIT can also be prevented on the power supply circuit connected to the anode of the CRT.
In FIG. 14, a point P2 on the IKB axis represents a current when a blue plane signal is input, for example. IKB reaches I1LIMIT because current flows into only the beam of the blue CRT although it is within the current limit range. As mentioned above, the rated current of IKB is roughly a third of I1LIMIT and thus in the example, a beam current about three times the rated value flows into the blue CRT. In a general image, the occurrence frequency of the blue plane signal is low. However, some machines such as videocassette recorders output a blue plane signal when the end of image reproduced on tape is reached and if such a machine is connected, a blue signal may be input consecutively. In this case, a rated or more beam current flows into the blue CRT for a long time. The occurrence frequency of a colored plane signal is low if the signal is limited to a television signal, but the occurrence frequency is considerably high if a menu screen and a computer-generated signal are contained in addition to the blue signal as a mute signal.
FIG. 15 shows the measurement result of the relationship between light emission intensity and light emission time of CRT when the CRT is driven with a beam current at a level close to the rated current. The light emission intensity is lowered with time and is lowered 10% or more after the expiration of 1000 hours. For example, if video projector is connected to a computer all the time, when the computer screen is a pale blue background, if it is input, the beam current of the blue CRT mainly flows, but the color is pale and thus the beam current is also distributed to IKR and IKG and the value of IKB does not reach I1LIMIT. In the example, the value of IKB becomes about a third of I1LIMIT, and under the condition that it is in the vicinity of the rated current of the CRT, lowering of the intensity of the blue CRT can be estimated as follows: If a computer is connected to the projector for four hours a day and the projector to which the computer is connected is operated for 300 days a year, the sum total of the CRT light emission time becomes 4×300 =1200 hours. From the result shown in FIG. 15, the light emission intensity of the blue CRT is lowered 10% or more as compared with the initial light emission intensity. Consequently, to display a white plane, blue is insufficient and the white plane is not displayed in the original color.
FIG. 16 shows a CRT beam current limiting circuit in a second related art. Components identical with those previously described with reference to FIG. 11 are denoted by the same reference numerals in FIG. 16. Numeral 7 denotes a current detection circuit for detecting cathode currents of CRTs, numeral 8 denotes a maximum value selection circuit for inputting a detected current, and numeral 9 denotes a maximum value selection circuit for inputting a plurality of detected currents. The operation of the components different from those previously described with reference to FIG. 11 will be discussed.
A beam current is detected as a potential drop when IA flows through a detection resistor R1 and a current detection circuit 5 outputs a detected voltage VDET1 to the maximum value selection circuit 9. On the other hand, the current detection circuit 7 detects the cathode currents of the color CRTs and outputs the detected cathode currents to the maximum value selection circuit 8, which then selects the maximum value from among the input detection values and outputs the maximum value to the maximum value selection circuit 9. The maximum value selection circuit 9 inputs the maximum detection value of the cathode currents and VDET1, selects the maximum detection value or VDET1, whichever is the greater, and outputs the selected one to a comparison circuit 6. When the beam current of any of the CRTs reaches a predetermined value, the detected cathode current is input through the maximum value selection circuit 8 to the maximum value selection circuit 9. The gain of the current detection circuit 7 is set so that detection value larger than the VDET1 is taken. The maximum value selection circuit 9, which inputs large cathode current detection signal, outputs it as VDET2 to the comparison circuit 6. The comparison circuit 6 sends excessive-level detection signal to a signal processing circuit 1, which then changes the contrast (or) brightness of a television signal, thereby lessening the average level of output R, G, and B signals. As a result, the average level of cathode currents IKR, IKG, and IKB is lessened and the beam current is limited.
FIG. 17 is a three-axis graph to show the range in which the cathode currents of the three CRTs are limited by the current limiting circuit in the second related art. The inside of a cube having one corner at an origin O with its opposite angle cut by TRI1 is the range in which the cathode currents can be changed. When a blue plane signal is input, IKB is limited to the current value indicated by P3 in the figure. IKR and IKG are also limited in a similar manner. When a white plane signal is input, limit is made to the current indicated by a point P1. The point P3 becomes drastically smaller as the value of IKB than a point P2, and for various signals, the beam current can be limited within the rated value of CRT.
FIG. 18 shows examples of a program guide menu displayed on a screen and signal waveforms. In the figure, PIC1 denotes a menus screen, A1 denotes a menu name display area, A2 denotes a menu content display area with white text on a blue background, A3 denotes a blue area as the menu background, V3 denotes a line representing a change in blue signal level with time for one scanning line of signals making up the menu, V4 denotes a line representing a change in red and green signal levels with time, and C1 denotes an area in which signal level changes finely corresponding to text. Since A3 is blue, the blue signal level is high in the whole time range of T2 and since the text is white, the case is almost the same as the case where blue plane signal is input. Red and green signals are at low level other than the text and become at large level only in the period of the text, but are at small level on average. When such a menu screen is input, the beam current is limited to the level indicated by P3 in FIG. 17.
Various menu screen colors are available and display time and frequency also vary. A menu consisting mainly of green and a menu having an area of a fundamental color in combination may be input. The beam current of each CRT is limited to a predetermined value for any combinations. Therefore, the light emission intensity of the CRT with a large beam current is lowered in accordance with the curve shown in FIG. 14 in response to the cumulative time of menu display. If a menu using areas of different colors in combination is used over a long period of time, the light emission intensity of the phosphor is also lowered for each color area. A similar problem also occurs in a computer signal and a blue mute signal in addition to the menu, as described above.
FIG. 19 shows a CRT beam current limiting circuit in a third related art. Components identical with those previously described with reference to FIG. 11 are denoted by the same reference numerals in FIG. 19. Numeral 30 denotes a calculation circuit and numeral 31 denotes a correction circuit. The operation of the components different from those previously described with reference to FIG. 11 will be discussed.
The calculation circuit 30 inputs a fundamental color signal before being amplified by an amplification circuit 2 and finds the beam current of each color CRT by calculation. If the calculation result indicating that an excessive current flows is found, the calculation circuit 30 outputs a detection signal to the correction circuit 31. Upon reception of VDET1, a signal of the result of detecting an anode current, the correction circuit 31 corrects the VDET1 based on the detection signal output by the calculation circuit 30 and outputs a control signal to a signal processing circuit 1.
According to the beam current limiting circuit in the first related art, the current of the total value of the anode currents is detected. Thus, for example, if a blue signal is input, the flow of the beam current largely exceeding the rated average current of CRT concentrates onto one blue CRT and the reliability of the parts is degraded or the parts are broken; this is a problem.
In the second related art example, the cathode current of each color and the total anode current are detected and are limited so that they are placed in predetermined ranges. Thus, the beam current can be limited so as not to exceed the rated value of the average beam current of CRT; however, if each cathode current is limited to a constant value across the board, the current of a normal image is also limited and the dynamic range of the beam current is lessened, decreasing the power of a display image; this is a problem.
In the third related art example, the cathode current itself is not detected and the beam current is estimated by calculation and thus current limiting cannot be executed with high accuracy because the actual beam current receives the effects of a gain error of the amplifier at the following state and a cutoff adjustment error of the beam current; this is a problem.
The following problem is common to the related art examples: A configuration method of a cutoff automatic adjustment circuit of CRT beam current is not disclosed and if a beam cutoff current detection circuit is configured independently of a circuit for detecting a large beam current, the circuitry becomes complicated and the costs are increased or the reliability is degraded.
The beam current limiting circuits in the related arts detect the average value or the peak value of currents and basically limit the current based on the signal waveform in the time range within several frames. In recent years, various signal sources have been displayed on a video projector and the occasions of producing screen display consisting mainly of text and graphics such as a program guide and a computer screen have occurred increasingly, in which case text and graphics are displayed in the screen consecutively over several ten frames as a fixed pattern. When a color area of a reasonable size is displayed stationarily like a menu screen, if the beam current of each CRT is less than the rated current, the current difference among the CRTs is large depending on the color arrangement of the screen and as the projector is used for a long period of time, the intensity of the phosphor area of the CRT with a large average current is degraded and a phenomenon in which a different color from the original image color is attached to a part or the whole of the screen, so-called screen phosphor burn-in occurs; this is a problem.